If you ever have to create cron expressions (I do while working with the Quartz Scheduler) these links may be helpful to you.
Firstly a basic cron expression tutorial for quartz (what they are, the basics and some examples)
some more examples of working with the cron format
And finally a useful website which can generate basic cron expressions for you and can be used to check that cron expression you have written do what you think they do.
Hope this helps